Cannabis is the most prevalent drug used by adults aged ?50 in the U.S., after alcohol and tobacco. Recent trends show dramatic increases in cannabis use among older U.S. adults, and cannabis use is particularly common in people living with HIV (PLWH). HIV has detrimental effects on both mobility and cognition, and similar to normal aging, mobility in PLWH may be influenced by cognitive function. In younger persons, cannabis use causes acute impairment in cognition and balance, but little is known about the combined effects of cannabis use and HIV infection on mobility, balance, and cognition, particularly among older individuals. HIV+ women have alarmingly high rates of falls and fractures, which occur at younger than expected age. Although women represent a sizeable proportion of the growing number of older PLWH, they are often underrepresented in HIV research. In the Women's Interagency HIV Study (WIHS), we found that 40% of women reported at least one fall over 2 years despite a mean age of 48 years, and current cannabis users had almost double the risk of having a fall among HIV+ but not HIV-women. The current proposal explores potential mechanisms underlying the increased fall risk associated with cannabis use, and examines whether acute effects of cannabis on mobility, balance, and cognition differ by HIV status in this aging population. We will compare the effects of controlled administration of active (7.0% THC) and inactive (0.0%) cannabis in aging HIV+ women on stable HAART and HIV- controls enrolled in the WIHS, on measures of balance, mobility, and cognition, including a cognitive-motor divided attention task (Walking While Talking, WWT). The WWT may help unmask subtle cognitive abnormalities by increasing the complexity of the walking condition, and predicts falls, frailty, disability, and mortality among older community-residing adults, yet has never been studied in cannabis users or PLWH. This proposal is nested in the WIHS, the largest and longest-duration cohort study of HIV-infected women in the U.S., and controls who are carefully selected to have similar demographics, socioeconomic factors, and lifestyle and behavioral risk profiles. Use of the established WIHS infrastructure allows us to examine mobility, balance, and cognition after cannabis administration as part of the current proposal, in relation to over 20 years of prospectively collected data on cannabis use patterns, and report of falls, ensuring completion of this exploratory study in a timely and efficient manner. An improved understanding of the effects of cannabis use on balance, mobility, and cognition and their relation to fall risk may have important implications for developing effective risk assessments and interventions for mobility impairment in aging women who use cannabis, with or without HIV.